1. Field of the Invention
The present invention relates to a tool holder on which a nonrotational tool of a machining center is mounted.
2. Description of the Related Art
While tools mounted on a spindle are automatically exchanged, a machining center that is used as one of machine tools can perform various kinds of machining by the respective tools. The spindle is rotated about a central axis by the drive of a motor. When machining performed by a rotating tool, such as drilling, tapping, or milling, is performed by the machining center, rotating tools such as a drill used in these kinds of machining are mounted on the lower end portion of the spindle through a tool holder and perform the machining of a workpiece by rotating while moving up and down together with the spindle.
On some machining centers, not only rotating tools but also nonrotational tools such as turning tools are mounted. A turning tool is a tool that performs lathe turning on the surface of a workpiece while the tip of the tool is pressed against the workpiece which is rotating. The turning tool itself needs to be fixed so as not to rotate.
Various techniques for stopping the turning of a nonrotational tool are disclosed as follows.
Japanese Patent Application Laid-Open (JP-A) No. 2005-329493 discloses a technique for stopping the turning of a turning tool, in which a disc-shaped flange is formed in a tool holder that holds a turning tool, keyways are formed on the outer periphery of the flange at two locations which face each other in a radial direction, and keys are provided in a spindle head that supports a spindle. In this technique, when a cutting tool is mounted on the spindle, the keys of the spindle head are engaged with the keyways of the flange of the tool holder, thereby restricting the rotation of the tool holder and preventing the turning of the turning tool.
JP-A No. 2012-213826 discloses a technique for stopping the turning of a turning tool, in which a turning preventing flange is formed on the outer periphery of a tool holder that holds a cutting tool so as to protrude to the outside in a radial direction, two engaging holes are formed in the turning preventing flange coaxial with the tool holder, and engaging protrusions are provided on a spindle head so that they can be engaged with these two engaging holes, respectively. In this technique, when the engaging protrusions provided on the spindle head are engaged with these two engaging holes, the turning of the turning tool is prevented.
Japanese Utility Model Application Laid-Open No. 5-70842 discloses a technique for stopping the turning of a turning tool, in which a pin is provided so that it protrudes from a tool holder that holds a turning tool, and a positioning block is provided on a spindle head so that it can be engaged with the pin. In this technique, when the positioning block is engaged with the pin, the turning of the turning tool is prevented.
JP-A No. 11-10485 discloses a technique for stopping the turning of a turning tool, in which couplings each having a plurality of teeth formed inside a spindle head are mounted on the outer periphery of a spindle and the inner periphery of the spindle head, respectively. In this technique, when the couplings of the spindle and those of the spindle head mesh with each other, the rotation of the spindle is restricted, thereby preventing the turning of a turning tool.
The techniques disclosed in the above-mentioned patent documents are employed as a countermeasure for preventing the turning of the tool when the tool holder is mounted on the spindle, a countermeasure for maintaining the posture of the tool, and a countermeasure for stiffness against a machining load. Here, machining, such as lathe turning using a nonrotational tool, is performed in a manner such that a spindle is not rotated in order to fix the nonrotational tool. However, a machining center is formed on the premise of the use of rotating tools. For this reason, if the machining center receives a large cutting load or repeated vibration during the stop of the spindle, a load is applied while bearings in the spindle are fixed. Accordingly, since only a part of the bearings are damaged, there is a concern that the life of the spindle may be shortened.
Further, when the tool holder is mounted on the spindle and an improper operation for starting the spindle is performed with the tool holder fixed without turning, there also is a concern that damage to the spindle may be caused.